If barely split beams have to be transported over several meters, they cannot be independently controlled with magnets, because the magnets act on both beams simultaneously. Bunch quality might be lost as a result, and loss of large amplitude components of beam—“beam halo”—can be aggravated. In addition, control of lattice tuning parameters such as beam envelope functions, phase advances, momentum compactions, and nonlinear aberrations may be compromised.
An example of two or more barely separated electron beams traveling through a common transport pipe includes a spreader/recombiner in which the angles of the split beams are very similar. This occurs in multi-pass systems where coaxial beams at different energies, but moving on a common axis, such as in a linac, need to be split into spatially separated beams for reasons such as recirculation transport. The lowest-energy beam determines the spreader's dipole field strength, and the high-energy beams typically end up very close together.
A dipole with a gradient provides only limited control of the individual split beams. The control is more difficult because control over the magnetic field is more limited. Dipoles have a far more significant impact on the geometry of the beam trajectories. If the field is varied over a wide range, the trajectories change a lot and this can create other interferences.
Accordingly, what is needed is a method for achieving independent control of multiple beams in close proximity to one another, such as in a spreader/recombiner. Independent control over the multiple beams would be of value in the design, construction, and operation of multipass SRF linac based accelerators, such as CEBAF, free electron lasers (FEL), or other energy recovery linacs (ERL).